The separation of natural gas streams by processes based upon the use of cryogenic techniques is known. Such processes basically consist of subjecting the natural gas stream to be separated to successively lower temperatures whereby the natural gas stream is separated or fractionated into its component parts in accordance with the boiling points of the various components. These components generally include, in descending order of their boiling points, hydrocarbons heavier than methane such as ethane and heavier hydrocarbon components, methane itself and nitrogen.
In general, most known natural gas separation processes comprise at least three distinct operative steps or stages. These include (1) a preliminary gas treatment step for the removal of water and acidic gases such as carbon dioxide and hydrogen sulfide, (2) a natural gas liquids product separation step using low but noncryogenic temperatures for the separation and recovery of the ethane and heavier hydrocarbon components and (3) a nitrogen separation or rejection step employing cryogenic temperatures for the separation and recovery of a methane-rich product stream useful as a high BTU content fuel source.
A typical natural gas treatment process is disclosed in U.S. Pat. No. 4,504,295. In this patent, a process is described which is directed to the separation of natural gas streams into a natural gas liquid product stream, a methane-rich product stream and a nitrogen-rich product or waste gas stream. The principal characteristic of the patented process is said to be the ability to effect the critical low temperature (i.e., cryogenic) separation at high pressures in a single high pressure distillation column. The refrigeration required to attain the cryogenic temperatures necessary to effect the critical separation at high pressures is provided by an auxiliary closed loop heat pumped refrigerant such as methane. This refrigerant is employed in indirect heat exchange relationship to various reflux and reboil streams flowing to and from the high pressure distillation column. According to the patent, the process provides the ability to deliver methane-rich and nitrogen-rich product streams at high pressures, thus avoiding the need for the supplemental compressor or pumping equipment required in more conventional low pressure processes.
In the more conventional low pressure processes, the low temperature refrigeration required for the critical separation is provided, at least in part, by expansion (i.e., use of the Joule-Thomson cycle) of the feed gas stream and/or various intermediate streams generated by such processes. As a result of using expansion, the final separation of the feed gas stream into methane-rich and nitrogen-rich product streams is carried out at or near atmospheric pressure. Being at or near atmospheric pressure, these product streams or at least the methane-rich product stream must be recompressed to pipeline or transportation pressures.
Illustrative of one such low pressure process for the separation of natural gas streams is the integrated process described in Energy Progress, Vol. 4, No. 4, December 1984, pp 214-221. In that process, the final critical separation of the natural gas stream, substantially free of its natural gas liquid constituents, into a methane-rich product stream and a nitrogen-rich product (or waste gas) stream is carried out in a low pressure distillation vessel operated at or near atmospheric pressure. The refrigeration required to effect the separation is provided by both indirect heat exchange and expansion of the intermediate product streams recovered from a high pressure distillation column operated immediately upstream of the low pressure column to effect a rough methane/nitrogen split. Finally, the separated and recovered methane-rich product stream is recompressed, by means of compressors or pumps, to elevated pressures.
The above references describe processes representing the extremes of known natural gas separation processes. It is clear that a process wherein neither auxiliary refrigeration nor supplemental compressor or pumping equipment is required would represent an advancement in the art.